A protective circuit of the above-mentioned kind is previously known from Swedish patent publication No. 358 509. In this circuit a special trigger circuit with auxiliary spark gaps, of the kind which is included in older surge arresters with silicon carbide varistors, is used. Since modern surge arresters are built up of zinc oxide varistors without series-connected spark gaps, the need of these spark gaps has been considerably reduced. This has led to a situation in which there is no longer any market base for a rational manufacture of such gaps. Therefore, also in protective devices of the kind to which the present invention relates, the aim is to avoid using auxiliry spark gaps.